The 50/1.8 is generally considered better IQ than the 35/2, but it's kinda soft at f/1.8, sharpens up nicely by f/3.5 or so.
I'd agree that it's better to consider the focal lengths used most, setting your zoom to one and playing around is the best way.
Or just split the difference and get the Shorty Mc40mm (but then you don't get faster than f/2.8).